Semi-submerged vessels, which were developed for operation at high sea states, also have been referred to in the prior art as Small Waterplane Area Twin HULL (SWATH) ships. Various configurations of these ships have been described in U.S. Pat. Nos. 3,623,444 and 3,897,744 issued to Thomas G. Lang; U.S. Pat. Nos. 4,552,083 and 4,557,211 issued to Terrence W. Schmidt; and Japanese Patent No. 52,987 issued Jan. 11, 1977.
All previous embodiments of semi-submerged vessels use an arrangement of elongated (small cross-sectional area to length) submerged hulls to provide the majority of the buoyancy. For efficient operation from the standpoint of powering and fuel consumption, SWATH, as with all displacement ships, are presently limited in speeds to those having a Froude number of less than 0.4.
Froude number (F) is defined as follows: ##EQU1## where v=speed
g=acceleration due to gravity PA1 l=length of hull. PA1 "as long a length as is compatible with other design requirements," Principles of Naval Architecture, Comstock, p. 345; PA1 "greater length will reduce wave-making resistance but increase the frictional resistance," Comstock, p. 342; and PA1 "vessels . . . are made as long and slender as practicable," Hoerner, p. 11-12.
The limit in speed of a displacement ship is best described in Modern Ship Design, by Thomas C. Gillmer, 1970 which states, "The practical limiting speed for displacement surface vessels is basically that of wavelength to ship length, where one wavelength, created by the ship, is equal to the ship's waterline length.
This, expressed quantatively, is V/.sqroot.L.apprxeq.1.3 (or F=0.39), and V is sometimes called the hull speed. When a surface ship attempts to exceed this speed it finds itself literally climbing a hill that it is creating. In exceptional cases of slim, highly powered ships such as destroyers, it is possible to exceed this speed, but it is seldom profitable."
The limitation in speed is primarily due to the large increase in wave resistance that occurs between a Froude number of 0.4 and 0.8. This increase in wave resistance is well established in the prior art for all surface displacement ships and is often referred to as the resistance or powering "hump." The hump is referred to as the "primary hump" in this application. See Fluid-Dynamics Resistance, by Sighard F. Hoerner, 1965. Because of the high wave resistance, operation in the "hump" speed region results in high propulsion power and inefficient fuel usage. According to Gilmer, supra, "A ship may be required to maintain a constant operational speed for long periods and it is clearly desirable that it should not do so at a hump on the Cw (wave drag) curve" (pg. 160). Normal operation for a displacement ship is at a Froude number corresponding to a "hollow" in the wave drag curve at a Froude number lower than the primary hump. The operational Froude number for various ship types is shown in FIG. 5.22 of Mechanics of Marine Vehicles, Clayton and Bishop, p. 220 and table A page 11-15, Hoerner, supra. Only the destroyer with its abundance of power operates at a Froude number above 0.4.
To delay the onset of high wave making resistance the prior art calls for:
Operation at a Froude number greater than 0.8 substantially reduces wave resistance. "The pressure distribution about a high speed vehicle is therefore quite similar to that about a vehicle progressing at a very low speed . . . This means that the wave making resistance of high speed vehicles (Fr.gtoreq.1.5, say) is small as it is for vehicles operating at very low speeds (Fr.ltoreq.0.15, say)" Clayton and Bishop, p. 219; however, to exceed the "hump" speed region requires excessive propulsion power for displacement (including SWATH) ships of the conventional form. The hull speed region at which this large increase in wave resistance occurs as a main or primary hump on the C.sub.w (wave drag) curve so as to result in a requirement for a maximum increase in propulsion power is defined as the "critical hump speed" in this application.
This critical hump speed is illustrated (for displacement ships of conventional design) by the legend "main hump" in the graph of FIG. 20 of the drawings of this application and is the speed at which a peak wave resistance occurs in the plot of wave resistance versus increasing hull speed as illustrated in FIG. 2 of the drawings of this application.
An object of the present invention is to provide a small waterplane area hull form which operates at reduced wave resistance so as to cause the critical hump speed to occur at a low hull speed where the available propulsion thrust is large enough to allow transition through the critical hump speed without excessive installed power and to thereby permit efficient operation to high speeds; that is, where the Froude number is greater than 0.8.